1. Field of the Invention
This invention relates to a semiconductor apparatus and a method for manufacturing the same.
2. Description of the Related Art
A vertical power MOSFET (Field Effect Transistor) is known as a MOSFET for high voltage operation. The power MOSFET has two important characteristics. One is an on-resistance, and the other is a break down voltage (withstand voltage). There is a trade-off between the on-resistance and the break down voltage. It is difficult for the power MOSFET to have a high break down voltage and a reduced on-resistance.
Super-junction devices are proposed to achieve the power MOSFET having a high break down voltage and a reduced on-resistance.
FIG. 17 is a cross sectional view of the semiconductor apparatus 100 having a super-junction structure (SJ structure). As shown in FIG. 17, the semiconductor apparatus 100 comprises a semiconductor substrate 101, a drift region 102, a base region 108, a source region 109, a gate oxide layer 106A, a gate electrode 107A, an intermediate insulating layer 110, a contact hole 110a, a source electrode 111, a column region 204 and a drain electrode 112.
The drift region 102 is formed on the semiconductor substrate 101, and operates as an electric field relaxation layer. The base region 108 is formed on the drift region 102. The source region 109 is selectively formed in the surface portion of the base region 108. The gate oxide layer 106A is formed on a surface of a trench formed in the base region 108. The gate electrode 107A is formed on the gate oxide layer 106A. The intermediate insulating layer 110 is formed over the gate electrode 107A and the source region 109. The contact hole 110a is formed in the intermediate insulating layer 110. The source electrode 111 is formed over the intermediate insulating layer 110, and the source electrode 111 is electrically connected to the source region 109 via the contact hole 110a. The column region 204 is formed in the drift region 102 under the base region 108. The drain electrode 112 is formed on the bottom surface of the semiconductor substrate 101.
The drift region 102 and the source region 109 have the same conductivity type as the semiconductor substrate 101 (for example, N or N+ type). The base region 108 and the column region 204 have an opposite conductivity type to the semiconductor substrate 101 (for example, P-type). The impurity concentration of the column region 204 is approximately the same as the impurity concentration of the drift region 102. The impurity concentration of the entire column region 204 is uniformed.
As shown in FIG. 17, the semiconductor apparatus 100 having the SJ structure is basically the same as a conventional vertical power MOSFET. The difference between the conventional power MOSFET and the semiconductor apparatus 100 is that the semiconductor apparatus 100 has the column region 204.
When a bias voltage is not applied between the gate electrode and the source electrode and a reverse bias voltage is applied between the drain electrode and the source electrode, depletion regions are extended from two junctions. One junction is a boundary between the drift region 102 and the base region 108, and the other junction is a boundary between the drift region 102 and the column region 204. The semiconductor device 100 becomes off state because the depletion regions are extended.
That is, the boundary between the drift region 102 and the column region 204 extends in depth direction. The depletion region between the drift region 102 and the column region 204 is extended in width direction, and the whole column region 204 and the drift region 102 are depleted when the width of the depletion region becomes wider than distance d shown in FIG. 17.
In case that the semiconductor apparatus 100 has the SJ structure and the distance d is sufficiently short, the break down voltage (withstand voltage) does not depend on an impurity concentration of the electric field relaxation layer. Therefore, the reduced on-resistance and the high break down voltage are achieved by the semiconductor apparatus 100 which has the SJ structure. Japanese unexamined patent publication No. 2001-298189 discloses the semiconductor apparatus having the SJ structure.
In the semiconductor apparatus having the SJ structure, the electric field concentrates at the PN junction under the base region 108 or the region under the gate oxide layer 106A when the reverse bias voltage is applied. Therefore, the characteristic of the gate oxide layer 106A is degraded because an avalanche current flows at above described regions. In case that the gate electrode 107A is formed in a trench as shown in FIG. 17, this problem often occurs.